Seeing the northern lights from Britain

A chance glimpse of the northern lights from your back garden is unlikely without some preparation, particularly if you live in Britain. This is where auroral prediction services such as Lancaster University’s AuroraWatch are useful.

Seismographs measure tremors in the Earth’s crust during earthquakes. In a similar way, magnetometers measure shudders of the Earth’s magnetic field due to disturbances in the electrical current systems that flow in the upper atmosphere. Lots fluctuations mean an auroral display is more likely.

Hundreds of magnetometers continuously monitor changes in magnetic field around the world, including the sub-auroral magnetometer network (SAMNET) run by AuroraWatch scientists.

The global data is collected together and an instantaneous assessment of the level of activity is generated, a bit like a weather forecast. Scientists issue aurora warnings for active ‘storm’ conditions that anyone can receive through the AuroraWatch email service.

A particularly useful predictor of the aurora that scientists use is the Kp index, also called the planetary-K index, which is compiled from magnetometer measurements at 13 stations that have records going back to 1949. Every 3-hour period of each day is assigned a Kp activity level between 0 and 9, somewhat similar to the Richter scale for earthquakes or the Beaufort scale for wind and storms. A Kp level of 0 means exceptionally quiet, whereas more disturbed times are described as:

    Kp = 4:    Calm Conditions
    Kp = 5:    Minor storm conditions
    Kp = 6:    Moderate storm conditions
    Kp = 7:    Strong storm conditions
    Kp = 8:    Severe storm conditions
    Kp = 9:    Extreme storm conditions

Kp levels of 0 to 4 are common; above 5 the Kp levels become increasingly rare, such that storms registering a Kp of 9 may only be observed a couple of times a year, or not at all during quiet periods of the solar cycle.

So how does this relate to our chances of seeing an aurora from a back garden in Britain? The answer is clearer if we take a look from space.  From the ground, aurorae look like delicate curtains or draperies with highly dynamic folds and curlicues.  But from space the higher vantage point reveals how the aurora forms rings that encircle the poles. 

When the Kp level is low the auroral rings are small and dim, contracted close to the poles. As magnetic activity increases the rings brighten and move towards lower latitudes, where they can be seen from the UK. 

Recently, Dr Steve Milan from the University of Leicester compared Kp activity to auroral ring characteristics using images taken over two years by NASA's Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. He created photo-fits of the auroral rings for different Kp levels (see picture). Each image shows the average auroral activity at a different Kp level, in a coordinate system centred on the northern magnetic pole.  Sunlight reflected from the day-lit atmosphere can be seen at the top of each image. In the auroral ring itself, blue and green indicates low auroral activity, while red represents intense aurora.  Superimposed in black are the coast-lines of the Northern Hemisphere, with the British Isles visible to the bottom right of each image.

The average ovals show that the Kp index has to approach ‘severe storm’ level 8 before an aurora can be seen from the Midlands, where Steve lives. Under cloudless conditions the aurora can be seen as a faint glow on the northern horizon at a maximum distance of 1000 kilometers.  Further away than this and they are hidden below the horizon by the curvature of the Earth. As the aurora come closer they appear higher in the sky and you can see their structure and dynamics more clearly.  Next time the Sun obliges with a solar storm, Steve will be eagerly scanning the northern skies.